Since polycarbonate is excellent in heat resistance, impact resistance and transparency, it has been widely used in many fields in recent years. Various studies have been carried out with processes for production of polycarbonate. Among them, polycarbonate derived from aromatic dihydroxy compounds such as 2,2-bis(4-hydroxyphenyl)propane, hereinafter “bisphenol A”, is industrialized by both processes of interfacial polymerization and melt polymerization.
According to the interfacial polymerization, polycarbonate is produced from bisphenol A and phosgene, but toxic phosgene has to be used. In addition, it remains a problem such as corrosion of equipments caused by by-products such as hydrogen chloride and sodium chloride and chlorine-containing compounds such as methylene chloride used in great quantities as a solvent, and difficulties in removal of impurities such as sodium chloride or residual methylene chloride which might have an influence on polymer properties.
Meanwhile, as a method for producing polycarbonate from an aromatic dihydroxy compound and diarylcarbonates, a melt-polymerization method has been long known, wherein, for example, bisphenol A and diphenylcarbonate are polymerized through a transesterification reaction under melting conditions as removing by-product aromatic monohydroxy compounds. Unlike the interfacial polymerization method, the melt-polymerization method has advantages such as not using solvents. However, it has an essential problem as follows: As the polymerization proceeds, viscosity of polymer in the system increases drastically to make it difficult to remove by-product aromatic monohydroxy compounds efficiently out of the system which would cause the reaction rate extremely decrease to make it difficult to increase the polymerization degree.
In order to solve the above problem, various attempts have been studied to extract aromatic monohydroxy compounds from polymer under conditions of high viscosity. For example, Patent Document 1 (Japanese Patent Kokoku No. S50-19600) discloses a screw-type polymerization vessel having a vent. Further, Patent Document 2 (Japanese Patent Kokai No. H02-153923) discloses a method using a thin-film evaporator in combination with a horizontal polymerization device.
Patent Document 3 (U.S. Pat. No. 5,521,275) discloses a method for redistribution of molecular weight of an aromatic polycarbonate under the presence of a catalyst using an extruder having a polymer seal and a vent under reduced pressure.
However, the methods disclosed in the above documents would not be able to increase the molecular weight of polycarbonate sufficiently. The above methods for increasing the molecular weight using catalyst in large quantity or using strict conditions such as applying a high shearing might cause problems which would have a significant influence to polymer such as the deterioration in hue or the progress of a cross-linking reaction.
It is known that the polymerization degree of polycarbonate can be increased by adding a polymerization accelerator in the reaction system of melt-polymerization. Increasing the molecular weight under a short reaction residence time and a low reaction temperature enables to increase the production volumes of polycarbonate which would make it easy to design simple and inexpensive reaction vessels.
Patent Document 4 (European Patent No. 0 595 608) discloses a method for reacting several diarylcarbonates at the time of redistribution which, however, would not bring a significant increase in molecular weight. Patent Document 5 (U.S. Pat. No. 5,696,222) discloses a method for producing a highly polymerized polycarbonate by adding a certain type of polymerization accelerator such as arylester compounds of carbonic acid and dicarboxylic acid including bis(2-methoxyphenyl)carbonate, bis(2-ethoxyphenyl)carbonate, bis(2-chlorophenyl)carbonate, bis(2-methoxyphenyl)terephthalate and bis(2-methoxyphenyl)adipate.
Patent Document 6 (Japanese Patent No. 4,112,979) discloses a method of reacting several salicylic carbonates with an aromatic polycarbonate in order to increase the molecular weight thereof.
Patent Document 7 (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-514754) discloses a method of introducing polycarbonate oligomer and bis-salicylic carbonate or the like into an extruder to increase in molecular weight.
Patent Document 8 (Japanese Patent No. 4286914) discloses a method of increasing the amount of terminal hydroxy groups by an active hydrogen compound such as a dihydroxy compound and subsequently to carry out a coupling reaction of the aromatic polycarbonate having the increased amount of terminal hydroxy groups using a salicylic acid ester derivative.
However, the method disclosed in the above document requiring the amount of terminal hydroxy groups of polycarbonate is complicated in processes because it needs both a reaction process with an active hydrogen compound and a reaction process with a salicylic acid ester derivative. In addition, according to the method, polycarbonate having many terminal hydroxy groups is low in thermal stability and has a risk of deterioration in physical properties. As shown in Non-Patent Documents 1-2, the increase in the amount of hydroxy groups by active hydrogen compounds might induce a partial chain-decoupling reaction accompanied by widening of the molecular weight distribution. Furthermore, relatively large amount of catalyst is required to obtain a sufficiently high reaction rate, which might bring about deterioration in physical properties at the time of forming processes.
Patent Document 9 (Japanese Patent Kokoku No. H06-94501) discloses a process for producing a high-molecular polycarbonate by introducing 1,4-cyclohexanediol. According to the method disclosed therein, however, 1,4-cyclohexanediol is introduced together with an aromatic dihydroxy compound into the polycondensation reaction system from the beginning and therefore, 1,4-cyclohexanediol would be consumed first by the polycarbonate bond-forming reaction to form an oligomer, and then the aromatic dihydroxy compound would be reacted to participate the highly polymerization reaction. For this reason, it has a such defect that the reaction time would become relatively long, which might cause the deterioration of appearance features such as the color or hue.
Patent Document 10 (Japanese Patent Kokai No. 2009-102536) discloses a process for producing polycarbonate by copolymerizing specific aliphatic diol and etherdiol. However, since the polycarbonate disclosed therein has an isosorbide skeleton as a main structure, excellent impact resistance required to aromatic polycarbonates would not be exhibited.
As mentioned above, the conventional methods for producing highly polymerized aromatic polycarbonate have many problems, and still there are requests for developing an improved production method which enables the increase in molecular weight of the aromatic polycarbonate resin satisfactorily while keeping good quality that the polycarbonate originally has.